104 CHEMICAL ENGINEERING The buckets of carriers representing a considerable item in the cost of bucket carriers, formulas for relative costs and relative depreciation are complicated by the spacing of the buckets, as well as by the size of buckets and the length of the carrier. For the standard spacings of 18, 24, 30 and 36 in., formulas (25) to (28) give results which are quite reliable, and if other spacings are desired interpolation may be resorted to and reasonably accurate results secured % RCbc> « Kwl(H + L)w' + O.OOSwZL -f 15.25 V^ Formula (25) Where, RCbc' Relative cost of bucket carrier with rigid buckets, K = Constant « 0.00278 for 18-in. spacing « 0.00208 for 24-in. spacing = 0.00167 for 30-in. spacing = 0.00139 for 36-in. spacing, H = Height to which load is elevated in feet, L = Horizontal travel of carrier in one direction in feet, wl = Width by length of bucket in inches, and w' = Weight of material handled in pounds per cubic foot. RCbu = Kwl(H + Z/X + (0.05L + 0.75) Vwl Formula (26) Where, RCbe'f = Relative cost of bucket carrier with pivoted buckets, and K = Constant = 0 . 00389 for 18-in. spacing = 0.00292 for 24-in. spacing * 0.00233 for 30-in. spacing = 0.00194 for 36-in. spacing RDbcl = Kwl(H + LX + O.WZwlL + 1.525 V^ Formula (27) Where, RDbe' = Relative depreciation (annual) of bucket carriers with rigid buckets, and K = Constant = 0.00066 for 18-in. spacing = 0 . 00050 for 24-in. spacing = 0 . 00040 for 30-in. spacing = 0.00033 for 36-in. spacing. RDbc» = Kwl(H + I/X + (0.005L -f 1.6) Vwl Formula (28) Where, RDbc» — Relative depreciation (annual) of bucket carriers with pivoted buckets, and K = Constant = 0 . 00072 for 18-in. spacing = 0 . 00054 for 24-in. spacing = 0 . 00043 for 30-in. spacing = 0.00036 for 36-in. spacing. On account of the numerous chain joints subject to the abrasive action of dust and fine particles of material, the depreciation of bucket carriers is much more rapid when in operation than when idle, so, provided the hours of operation per year number 60 per cent or more of the available annual working hours — usually taken as 2,500 — depreciation per year is customarily taken as chargeable only during actual working hours. Fixed charges are figured at 8.5 per cent of the initial cost of the equipment per year, while the labor expense averages close to 6 cts. per hour of operation for each inch of bucket width for carriers with rigid buckets and 5 cts. for carriers with pivoted buckets. Incidental expenses for lubricants, etc. average close to 1H cts. per horsepower consumed.